4-substituted 3-hydroxybutyric acid derivatives and vicinal cyano, hydroxy substituted carboxylic acid esters are commercially important intermediates in the synthesis of pharmaceuticals. Nonracemic chiral 4-substituted 3-hydroxybutyric acid esters may be utilized in the synthesis of HMG-CoA reductase inhibitors, such as atorvastatin, fluvastatin, rosuvastatin, and itavastatin. For example, an ester of (R)-4-cyano-3-hydroxybutyric acid and an ester of (3R,5R)-6-cyano-3,5-dihydroxyhexanoic acid are key intermediates for the production of the cholesterol lowering agent atorvastatin. Methods have been described for producing certain 4-substituted 3-hydroxybutyric acid esters. Isbell, et al., Carbohydrate Res., 72:301 (1979), report a method for synthesizing an (R)-4-cyano-3-hydroxybutyric acid ester by reacting the monohydrate calcium salt of threonine with hydrogen bromide to produce a dibromo derivative of threonine, which is then converted to a vicinal bromohydrin. The hydroxyl group of the bromohydrin is protected prior to reaction with sodium cyanide.
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Acta Chem. Scand., B37, 341 (1983) reports a method for producing a 4-cyano-3-hydroxybutyrate from a 4-bromo-3-hydroxybutyrate that requires protecting the hydroxy group with a protecting group prior to reaction with sodium cyanide. Recent routes to synthesize 4-cyano-3-hydroxybutyrate esters involve the uncatalyzed chemical reaction of a 4-bromo- or 4-chloro-3-hydroxybutyrate ester, without protection of the hydroxyl group, with a cyanide salt. By-products, however, are formed under the basic conditions created by the basic cyanide anion, which are particularly problematic to remove from the product. 4-Cyano-3-hydroxybutyrate esters are high boiling liquids and vacuum fractional distillation is required to separate the 4-cyano-3-hydroxybutyrate ester from these by-products. The distillation conditions are prone to generate additional by-products and the distillation is troublesome to operate successfully.
The use of a 4-chloro-3-hydroxybutyric acid ester as a starting material in the synthesis of a 4-cyano-3-hydroxybutyric acid ester is more economically attractive than the use of a 4-bromo-3-hydroxybutyric acid ester, but requires more forcing conditions in its reaction with cyanide salts due to the lower reactivity of the chloro substituent compared to the bromo substituent. While the cyanation of 4-chloro-3-hydroxybutyrate esters proceeds with alkali cyanide and high temperature, these forcing conditions lead to substantial by-product formation, requiring extensive isolation and purification procedures that result in additional yield loss. U.S. Pat. No. 5,908,953 discloses that, besides unreacted starting material, crude lower alkyl esters of (R)-4-cyano-3-hydroxybutyric acid may contain hydroxyacrylate, cyanoacrylate, 3-cyanobutyrolactone, 3-hydroxybutyrolactone, γ-crotonolactone, 3-cyano-4-hydroxybutyrate lower alkyl ester, 3,4-dicyanobutyrate lower alkyl ester and high-boiling uncharacterized compounds. U.S. Pat. No. 5,908,953 further describes a purification method for lower alkyl esters of (R)-4-cyano-3-hydroxybutyric acid that involves distillation of a crude mixture in the presence of a solvent that has a boiling point of 50° C. to 160° C. at 10 Torr. Using such distillation methods, the decomposition of unreacted starting material is said to be minimized, which otherwise can result in a dramatic overall loss in (R)-4-cyano-3-hydroxybutyric acid lower alkyl ester production. U.S. Pat. No. 6,140,527 describes an alternative approach for treating crude lower alkyl esters of (R)-4-cyano-3-hydroxybutyric acid that involves removal of the dehydrated by-products, such as 4-hydroxycrotonic acid esters, by chemical reaction, which renders these components water soluble and extractable. Thus, although these methods utilize a readily available starting material, significant yield loss and product purification requirements make them commercially undesirable. Accordingly, more efficient methods for producing nonracemic chiral 4-substituted 3-hydroxybutyric acid esters under milder conditions would be highly desirable.
Halohydrin dehalogenases, also referred to as haloalcohol dehalogenases or halohydrin hydrogen-halide lyases, catalyze the elimination of hydrogen halide, as proton and halide ion, from vicinal halohydrins to produce the corresponding epoxide. These enzymes also catalyze the reverse reaction. Nagasawa et al., Appl. Microbiol. Biotechnol. vol. 36 (1992) pp. 478–482, disclose activity of a certain halohydrin hydrogen-halide lyase on 4-chloro-3-hydroxybutyronitrile among other vicinal halohydrins. Nakamura et al., Biochem. Biophys. Research Comm. vol. 180 (1991) pp. 124–130 and Tetrahedron vol. 50 (1994) pp 11821–11826, disclose activity of a halohydrin hydrogen-halide lyase to catalyze the reaction of certain epoxides with cyanide to form the corresponding beta-hydroxynitriles. In these references and U.S. Pat. No. 5,210,031, Nakamura et al. disclose a reaction of epihalohydrin with alkali cyanide in the presence of a certain halohydrin hydrogen-halide lyase to produce the corresponding 4-halo-3-hydroxy-butyronitrile. In U.S. Pat. No. 5,166,061, Nakamura et al. disclose a reaction of a 1,3-dihalo-2-propanol with alkali cyanide in the presence of certain dehalogenating enzymes to produce the corresponding 4-halo-3-hydroxybutyronitrile. In Tetrahedron vol. 50 (1994) pp 11821–11826, Nakamura et al. disclose the reaction of 1,3-dichloro-2-propanol with cyanide using a purified halohydrin hydrogen-halide lyase to produce 4-chloro-3,-hydroxybutyronitrile.
Lutje-Spelberg et al., Org. Lett., vol. 2 (2001) pp 41–43, discloses activity of a halohydrin dehalogenase to catalyze the reaction of certain styrene oxides with azide to form the corresponding 1-phenyl-2-azido-ethanol.